Research overview

Cell differentiation is driven by co-ordinated changes in the gene expression profile of the cell: some genes are switched on, others are switched off. The mature sperm is a highly specialised cell (nearly 2mm long), whose formation from a simple primary spermatocyte involves meiosis to form round spermatids, followed by complex changes in cell architecture to form the final elongated motile sperm. During spermatogenesis there is a dramatic switch in the gene expression profile of male germ-line cells: as they enter the primary spermatocyte stage they activate transcription of a large set of genes required for sperm production. We have identified a set of proteins, encoded by the meiotic arrest genes, that work together to activate this transcriptional programme, and are investigating the composition, activity and evolution of this complex. We recently discovered that another small set of genes is transcribed after meiosis, and that these late transcribed mRNAs localise to a discrete region of the cell. We are studying their transcriptional control, and the mRNA localisation mechanism. Continued production of sperm is maintained via a stem cell system, and we are characterising a transcription factor required for stem cell maintenance.

I graduated from Cambridge University with a BA in Natural Sciences (Zoology) in 1990, then went to Dundee University to carry out PhD research on the regulation of cell division in fruit flies. In 1995 I went to Stanford University in California for a period of post-doctoral research, concentrating on the role of specific genes co-ordination of various cellular events during sperm production in flies. In 1998 I moved to Oxford to set up my own lab, initially as a departmental lecturer, and more recently (since 2001) as a Royal Society University Research Fellow. I continued to focus on spermatogenesis in Drosophila, specifically looking at regulation and function of testis specific genes. In April 2008 I moved to Cardiff University, to take up a position as a Senior Lecturer, continuing with the fly testis research.

Regulation of Gene Expression in Drosophila Spermatogenesis

Cell differentiation is driven by co-ordinated changes in the gene expression profile of the cell: some genes are switched on, others are switched off. One of the most astonishing developmentally regulated changes in cell morphology occurs in spermatogenesis. The mature sperm is a highly specialised cell, whose formation from a simple primary spermatocyte involves an unusual cell division (meiosis) to form a round spermatid, followed by complex changes in the cell architecture to form the final elongated motile sperm. These differentiation events require many gene products used at no other time in development. Underlying this there is a dramatic switch in the gene expression profile of male germ-line cells: as they enter the primary spermatocyte stage they activate transcription of a large set of genes required for sperm production. We have recently discovered that another small set of genes is transcribed after meiosis, in elongating spermatids, and are investigating their role in sperm function.

The meiotic arrest loci

The meiotic arrest class of Drosophila genes regulate transcription in spermatogenesis; specifically they are required for activation of expression of numerous genes required for spermatid differentiation. A series of micro-array experiments revealed that approximately half of all Drosophila protein coding genes are expressed in testes, and that 15-20% of these are regulated by our genes, ie, the meiotic arrest genes control transcription of up to 10% of all Drosophila protein coding genes. We cloned and characterised five of these meiotic arrest genes (aly, comr, achi/vis,topi and tomb). aly and comr have unknown functions, topi, tomb and achi/vis encode DNA binding proteins. In normal primary spermatocytes all the meiotic arrest proteins are chromatin associated, consistent with their role in gene-expression regulation. Their localisation in different mutant backgrounds varies: e.g. aly function is required for the nuclear localisation of Comr, and vice versa, indicating that formation and localisation of an active complex is highly regulated. We are currently investigating the formation of the complex, and its activity at target promoters.

Gene function in spermatogenesis

In 1998 I drew a model of the function of the meiotic arrest genes, in which I postulated that aly would regulate many genes required for spermatid differentiation. This model was based on expression data for about 20 genes. The array experiment was designed to test the model with many more genes. However, the main observation revealed by the array analysis is that we know very little about gene function in Drosophila spermatogenesis. I could initially not answer my question "do the meiotic arrest loci predominantly regulate genes required for spermiogenesis, and not genes required before meiosis?" because the function of the majority of genes that changed significantly in the mutants was not known. We therefore began a large-scale functional-genetics project in which we are using RNA in situ hybridisation to describe the expression pattern, and genetic regulation, of more than 1000 genes in testes. We have examined the expression patterns of about 1200 genes, and it seems my initial prediction was correct – genes that act after meiosis tend to be regulated by the meiotic arrest genes, whereas genes that act earlier tend not to require the meiotic arrest genes for their expression.

Post-meiotic gene expression in Drosophila

It has been generally accepted that there is no post-meiotic transcription in Drosophila spermatogenesis. However, in our in situ hybridisation project, we have identified about 25 genes that are transcribed during the elongation stage of spermatid development. Moreover, their transcripts are localised to the distal ends of the spermatids. The localised transcripts fall into two classes - "cups" and "comets". A mutation in one of the comet genes, scotti, is male sterile, with defects late in spermatogenesis. We are using in vivo reporter constructs and genetic analysis to examine the mechanisms controlling the mRNA and protein localisations and functions.

Funding

Research in my lab is funded by the Royal Society, The Wellcome Trust and BBSRC.